Timing is important here, as you want to get the drama of two separate layers with gradation between. Let the first layer set partially but not completely--if you add the blackberry layer too soon, it will muddle your color; a bit too late, and it will slide right off the bottom layer.
1 1/2 cups vodka, divided
4 envelopes unflavored gelatin, divided
1 1/2 cups sugar, divided
1 cup fresh lemon juice
1 cup water, divided
8 cups blackberries
Lemon rind strips (optional)
How to Make It
Place 1 cup vodka in a medium bowl. Sprinkle 2 envelopes gelatin evenly over vodka; let stand 3 minutes. Combine 1 cup sugar, lemon juice, and 1/2 cup water in a medium saucepan over medium-high heat. Bring just to a boil, stirring until sugar dissolves. Add hot lemon juice mixture to vodka mixture, stirring until gelatin dissolves. Pour lemon-vodka mixture into an 8-inch square glass or ceramic baking dish coated with cooking spray; chill 1 hour and 45 minutes.
Place remaining 1/2 cup vodka in a medium bowl. Sprinkle remaining 2 envelopes gelatin evenly over vodka; let stand 3 minutes. Combine remaining 1/2 cup sugar, remaining 1/2 cup water, and 8 cups blackberries in a large saucepan; bring to a boil. Reduce heat; simmer 10 minutes, stirring mixture occasionally. Strain blackberry mixture through a fine sieve over a 4-cup glass measuring cup; discard solids. Add 2 cups hot blackberry mixture to vodka mixture, stirring until gelatin dissolves (reserve remaining blackberry mixture for another use). Gradually pour blackberry-vodka mixture evenly over partially set lemon-vodka mixture. Chill 3 hours or until set. Cut into 36 cubes. Garnish with blackberries and lemon rind strips, if desired.